Industrial coatings are surface protective coatings (paint coatings) applied to substrates and typically heat cured to form continuous films for decorative purposes as well as to protect the substrate. A protective coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives, where the polymeric binder acts as a fluid vehicle for the pigments and imparts rheological properties to the fluid paint coating. Upon curing, the polymeric binder hardens and functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paint coatings contain little or no opacifying pigments and are described as clear coatings. The manufacture of paint coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in the polymeric binder, and thinning to commercial standards.
Epoxy resins are particularly desirable for use in protective surface coating materials as a vehicle or polymeric binder for pigments, fillers, and other additives where the epoxy resins advantageously provide toughness, flexibility, adhesion, and chemical resistance. Water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions and particularly useful for interior surfaces. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity wherein can coatings must not alter the product taste of canned beverages. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by absorption of flavor by the coating, or sometimes by chemical reaction, or by some combination thereof.
Container coating technology frequently utilizes an epoxy resin which has been grafted with acrylic monomers, styrene, and methacrylic acid. This grafted epoxy resin is prepared in solvent, usually butyl cellosolve, and n-butanol, to maintain low processing viscosities and then reduced with water by a direct or inverse let down procedure. Although cured film properties are highly desirable, such coatings suffer from the fact that sizeable amounts of organic solvent are required to obtain good performance. High molecular weight epoxy resins typically require 50% to 90% solvent (based on total solids plus organic solvent) before reducing with amine and water. Epoxy based can coatings comprising a carbon grated acrylic chain produced in the presence of an extender resin are disclosed in U.S. Pat. No. 4,399,241 and U.S. Pat. No. 4,482,671 while U.S. Pat. No. 4,595,716, and U.S. Pat. No. 5,157,078 teach a carbon grafting process involving solvent polymerization at moderate temperatures with high levels of peroxide initiator to produce a carbon-graft polymer. The high solvent levels, however, invariably carry over to the aqueous dispersion when the resulting polymers are dispersed into water to produce a VOC (volatile organic compounds) level considerably above 2 and typically between 3 and 4 pounds volatile organic compounds per gallon of resin solids.
Commonly assigned U.S. Pat. No. 5,290,828 discloses an acrylic grafted epoxy polyester terpolymer produced by in-situ copolymerization of ethylenic monomers with low molecular weight epoxy and unsaturated polyester resins where carboxyl monomers esterify epoxy groups while monomer double bonds coreact with polyester double bonds to form the terpolymer and subsequently dispersed into water. Commonly assigned Ser. No. 222,029 filed Apr. 4, 1994 U.S. Pat. No. 5,464,885 discloses an acrylic grafted epoxy-ester produced by first esterifying epoxy resin with a carboxyl functional unsaturated polyester to form an unsaturated epoxy-ester. The unsaturated epoxy-ester is dispersed into water and followed by in-situ copolymerization of ethylenic monomers in the aqueous dispersion, where the copolymerized monomers partially graft to the preformed unsaturated epoxy ester. Commonly assigned U.S. Ser. No. 274,804 filed Jul. 14, 1994 discloses a carboxyl functional acrylic copolymer dispersed into water to provide a polymeric dispersant and enable subsequent aqueous dispersement of a high molecular weight carboxyl functional polyester into water. The aqueous dispersion of acrylic copolymer and polyester are subsequently crosslinked with a dispersed diepoxide resin to produce microgel emulsion particles. Coating compositions utilizing microgels are shown in U.S. Pat. No. 4.897,434 where epoxy is first reacted with a carboxyl addition polymer in the absence of water to form a non-aqueous epoxy ester, then dispersed into water, followed by self-crosslinking between epoxy groups and carboxyl groups in the preformed epoxy ester.
It now has been found that epoxy based coating compositions exhibiting excellent film resistance properties can be successfully prepared without the use of organic solvent during synthesis of the polymeric binder for the coating. In particular, it has been found that low molecular weight, low viscosity linear polyesters can be mixed with and used as a diluent for high molecular weight epoxy resins to provide a fluid medium for in-situ copolymerization of ethylenic monomers including carboxyl monomers in the presence of the polyester diluent and the high molecular weight epoxy to produce an addition copolymer, preferably carbon grafted to the epoxy, and the polyester diluent. The resulting mixture is subsequently dispersed into water assisted by a volatile base to produce a water dispersed polymeric composition of polyester, epoxy, and addition copolymer. After dispersion into water, low molecular weight liquid diepoxide resin is added to the water dispersed polymers and then coreacted with carboxyl functionality on the addition copolymer to produce aqueous dispersed crosslinked microgel polymer particles. Coating compositions containing the microgel crosslinked polymer as the polymeric binder can be heat cured to provide excellent clear, smooth and glossy paint films without organic volatile emissions. Boiling water resistance of the cured film is excellent while damage resistance is improved due to modifying characteristics of the polyester diluent. Plasticizing effects imparted by the low molecular weight polyester diluent further reduces or eliminates the need for the inclusion of low Tg ethylenically unsaturated monomers in the addition copolymer. These and other advantages of the invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.